1. Field of the Invention
The present invention relates to charge transfer devices used in solid state image elements or signal delay devices, and more particularly to a method of making a charge transfer device having a plurality of transfer electrodes made of a single conductive layer.
2. Description of the Prior Art
Generally, charge transfer devices and signal delay devices use charge transfer devices which transfer sequentially charges of video signals in one direction by utilizing a potential difference. Such devices for transferring charges of video signals comprise a plurality of uniformly spaced transfer electrodes arranged on a silicon substrate by means of an insulating layer. By a drive voltage applied to each transfer electrode, a charge accumulated in a channel defined at the upper portion of silicon substrate flows in one direction toward an adjacent transfer electrode.
Now, conventional methods of making charge transfer devices will be described in conjunction with FIGS. 2a to 3d.
The method illustrated in FIGS. 2a to 2f uses a polycrystalline silicon material as a charge electrode material and a thermal oxidation process for providing an electrical isolation between adjacent transfer electrodes, capable of making the gap between adjacent transfer electrodes sufficiently small.
In accordance with the method, on a P type silicon substrate 1 are formed a N type impurity diffusion layer 2, a silicon oxide film 3 and a first polycrystalline silicon layer 4 doped with an impurity, in this order, as shown in FIG. 2a. The first polycrystalline silicon layer 4 is then selectively etched using a reactive ion etching (RIE) method under the condition that a photoresist (not shown) is used as a mask, so as to form a first polycrystalline silicon pattern 4a as a first transfer electrode, as shown in FIG. 2b.
Thereafter, the exposed portion of silicon oxide film 3 which was subjected to an etching damage upon performing the RIE method is etched under the condition of using the first polycrystalline silicon pattern 4a as a mask, as shown in FIG. 2c. As a result, the silicon oxide film 3 remains only at its portion disposed beneath the first polycrystalline silicon pattern 4a.
Another silicon oxide film 5 is formed over the resultant entire exposed surface, that is, the exposed portions of the N type impurity diffusion layer 2 and the first polycrystalline silicon pattern 4a, by using the thermal oxidation method, as shown in FIG. 2d. Over the silicon oxide film 5 is then formed a second polycrystalline silicon layer 6 doped with an impurity.
As shown in FIG. 2e, the second polycrystalline silicon layer 6 is selectively etched for forming a second polycrystalline silicon pattern 6a as a second transfer electrode.
Subsequently, the exposed surface of the second polycrystalline silicon pattern 6a is thermally oxidized, thereby causing a silicon oxide film 7 to be formed, as shown in FIG. 2f. Over the silicon oxide film 7 is formed a smoothing boron phosphorous silicate glass (BPSG) layer 8.
In accordance with the above-mentioned method, it is possible to achieve a small gap of not more than 0.2 .mu.m between adjacent transfer electrodes. This is because the gap defined between the first and second transfer electrodes depends on the thickness of the silicon oxide film 5 formed by the oxidation of the first polycrystalline silicon pattern 4a as the first transfer electrode. Accordingly, it is possible to achieve a state of hardly generating a potential pocket caused by the gap between the first and second transfer electrodes.
This method utilizing a double-layered polycrystalline silicon layer is suitable for achieving a sufficiently small transfer electrode gap, in that a high transfer efficiency is achieved. In this regard, most of currently commercial signal transfer devices utilize the method.
In devices handling minute signal charges such as solid state image elements, use of the above-mentioned method may cause certain problems.
For example, it is needed to provide two conductive layers for forming transfer electrodes, resulting in a complexity in manufacture. Moreover, the exposure of the thermally oxidized surface of silicon substrate increases a danger that the surface is contaminated by impurities. By such a contamination, a local abnormality in potential occurs beneath transfer electrodes, thereby causing video signal charges to be coupled. Such a coupling prevents an accurate transfer of signal charges.
It is also needed to provide a certain overlap space for assuring an overlap between the first and second transfer electrodes made of different conductive materials. Due to such an overlap space, there is a limitation on reducing the area occupied by transfer electrodes.
In addition, the overlap between the first and second transfer electrodes increases the layer capacitance between the first and second transfer electrodes to which different clock pulses are applied, resulting in unnecessarily increasing the consumption of electric power for driving signal charge transfer devices.
The use of polycrystalline silicon for the first and second transfer electrodes causes a degradation in performance in the signal charge transfer devices. In charge coupled devices of a frame transfer type having a structure in which incident lights corresponding video signals pass through transfer electrodes made of polycrystalline silicon, the sensitivity to blue color is decreased due to the characteristic of polycrystalline silicon absorbing lights of a short wave-length, thereby causing the spectral sensitivity charactersitic to be distorted.
Since the polycrystalline silicon has too high resistance to be used for transfer electrodes, it is unsuitable for signal charge transfer devices for a high speed driving. Accordingly, these devices should have a transfer electrode construction made of a material having a transmittivity higher than that of polycrystalline silicon and a electrode material having a lower resistance.
On the other hand, FIGS. 3a to 3d illustrate a method for forming transfer electrodes by using a single polycrystalline silicon layer.
In accordance with the method, on a P type silicon substrate 11 are first formed a N type impurity diffusion layer 12, a silicon oxide film 13 and a first polycrystalline silicon layer 14 doped with an impurity, in this order, as shown in FIG. 3a. Over the first polycrystalline silicon layer 14, a photoresist pattern 15 is then formed which corresponding to each transfer electrode to be formed, as shown in FIG. 3b. Using the photoresist pattern 15 as a mask, the first polycrystalline silicon layer 14 is then selectively etched using an anisotropy etching method such as the RIE method, so as to form a first polycrystalline silicon pattern 14a a first transfer electrode, as shown in FIG. 3b.
Thereafter, the photoresist pattern 15 is removed, so as to expose the surface of the first polycrystalline silicon pattern 14a which is, in turns, subjected to a thermal oxidation, thereby forming a silicon oxide film 16, as shown in FIG. 3c. Over the silicon oxide film 16 is formed as BPSG layer 17 for providing a surface smoothness, as shown in FIG. 3d.
This method has the following advantages:
First, the manufacture is easy;
Second, the manufacture cost is inexpensive;
Third, there is no contamination by impurities because of no exposure of the silicon substrate and no occurrence of a damage caused by the thermal oxidation;
Fourth, the capacitance between transfer electotrodes is reduced; and
Finally, there is no limitation on the material of transfer electrodes.
On the other hand, the method has the following disadvantage:
That is, it is difficult to a sufficient narrow transfer electrode gap to obtain a high charge transfer efficiency, taking into consideration the current semiconductor processing techniques on which the size of gap depends.
In charge transfer devices for a high drive speed in which drive pulses have a frequency of a MHZ band, a two-phase drive type charge coupled device (CCD) is generally adapted to obtain a higher charge transfer efficiency. In making such a two-phase drive type CCD, a self-aligning method is important.
Now, a conventional method of making a two-phase drive type charge transfer device using a self-alignment will be described.
In accordance with the method, on a P type silicon substrate 21 are formed a N type impurity diffusion layer 22, a silicon oxide film 23 and a first polycrystalline silicon layer 24 doped with an impurity, in this order, as shown in FIG. 4a. The first polycrystalline silicon layer 24 is then selectively etched using the RIE method under the condition that a photoresist 25 is used as a mask, so as to form a first polycrystalline silicon pattern 24a as a first transfer electrode, as shown in FIG. 4b.
After removing the photoresist pattern 25, the exposed portion of silicon oxide film 23 which was subjected to an etching damage upon performing the RIE method is etched under the condition of using the first polycrystalline silicon pattern 24a as a mask, as shown in FIG. 4c. As a result, the silicon oxide film 23 remains only at its portion disposed beneath the first polycrystalline silicon pattern 24a.
Thereafter, the resultant entire exposed surface is subjected to a thermal oxidation for forming another silicon oxide film 26 over the surface of N type impurity diffusion layer 22 and the surface of first polycrystalline silicon pattern 24a, as shown in FIG. 4d. Under the condition of using the first polycrystalline silicon pattern 24a as a mask, a P type impurity is implanted in the N type impurity diffusion layer 22, so as to form a P type impurity layer 27. Accordingly, the formation of P type impurity layer 27 is achieve in a self-aligned manner, without using any additional mask.
Over the resultant entire exposed surface is then formed a second polycrystalline silicon layer 28 doped with an impurity, as shown in FIG. 4e. As shown in FIG. 4f, the second polycrystalline silicon layer 28 is selectively etched for forming a second polycrystalline silicon pattern 28a as a second transfer electrode. Subsequently, the exposed surface of the second polycrystalline silicon pattern 28a is thermally oxidized, thereby causing a silicon oxide film 29 to be formed. Over the resultant entire exposed surface is formed a BPSG layer 30 as a smoothing layer.
FIG. 4g illustrates a wired condition of the obtained signal charge transfer device wherein adjacent first and second polycrystalline silicon patterns 2a and 28a are connected with each other by means of wires, so as to apply two-phase drive clock signals .sub.1 and .sub.2 to each other alternately.
The self-aligning process used in the case of FIGS. 4a to 4g is not used in the case of FIGS. 3a to 3d. As a result, where a P type impurity region or layer is formed at the N type impurity diffusion layer 12 so as to provide a potential well in the latter case, an arrangement shown in FIGS. 5a to 5c may be formed. In FIGS. 5a and 5c, the P type impurity layer is designated by the reference numeral 18.
In case shown in FIG. 5a, when two-phase drive clock signals .sub.1 and .sub.2 are applied, a potential barrier B occurs unnecessarily due to the space between the first polycrystalline silicon pattern and the P type impurity layer 18, as shown in FIG. 5b. In case shown in FIG. 5c, a potential pocket P occurs unnecessarily when two-phase drive clock signals .sub.1 and .sub.2 are applied, as shown in FIG. 5d. as a result, the signal charge transfer efficiency is considerably reduced.
As apparent from the above description, the conventional method using a double-layered polycrystalline silicon structure for transfer electrodes involves many difficulties in manufacture and limitations on designs of charge transfer devices, while achieving a narrow transfer electrode gap.
On the other hand, the conventional method using a single polycrystalline silicon layer for transfer electrodes achieves hardly a narrow transfer electrode gap due to the limitations of photoing and etching techniques. Moreover, the method for forming single-layered transfer electrodes has a disadvantage that the self-aligning process for making a two-phase drive type CCD can not be used.